(CNN) -- The Army has joined an investigation of the death of a soldier who was seven months pregnant, and local police are treating the case as a homicide.

Spc. Megan Lynn Touma, 23, was found dead June 21 in a hotel near her base in the Fort Bragg area in North Carolina, authorities said. Her decomposing body was in a bathtub, according to a search warrant obtained by CNN.

In a news release Monday, Fayetteville Police Department officials said they are treating the death as a homicide and have asked "special agents from the U.S. Army Criminal Investigation Command" to join the investigation. Army officials joined the probe Friday, authorities said.

Officials said they have identified a fellow soldier as a "person of interest" in the case. Lt. Col. John Clearwater, an Army Special Operations Command spokesman, told CNN affiliate WRAL that the person of interest is a training student at the John F. Kennedy Special Warfare Center and School at Fort Bragg.

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One question is whether Touma had been dead for more than a week without the Army reporting her missing. Army officials contacted by CNN said it is policy to declare a soldier AWOL if he or she has been missing for 24 hours. This did not happen in Touma's case.

The last time Touma was accounted for was at a roll call June 12, the day she arrived from a stint in Germany, said the Army, which has no documentation that she arrived for her next scheduled roll call June 16 and is investigating why she was not properly accounted for.

"The Army knows that she stayed in that hotel," he said. "And then they didn't even bother to check on her and make sure she was OK?"

Reporters have questioned officials about the identity of the father of Touma's child and whether that person is a suspect in the case. Investigators declined to answer. Touma's aunt has said the soldier had a boyfriend, but the aunt did not know if that man was the father.

On Saturday, The Fayetteville Observer reported that it had received a letter from someone purporting to be Touma's killer. The newspaper says police are also investigating the letter, which is dated June 17.

The letter included the name of the hotel, room number and date Touma was found. It also mocked Fayetteville police, saying, "I basically, sat there and watch while investigaters were on site." "Investigators" was misspelled in the letter.

The author also wrote, "It was a master piece. I Confess, that I have killed many times before in several states, but now I will start using my role-model's signature. There will be many more to come."

Though authorities have said they will investigate the letter, it was not mentioned in Monday's news release.

The letter has a circle with a cross in it, the same symbol used by the Zodiac Killer, who was responsible for at least five deaths in California in 1968 and 1969. The Zodiac Killer was never identified. Read the letter »

The Observer reported on Friday that a source close to the investigation said that the same symbol was drawn in lipstick on a mirror in Touma's motel room.

Lt. David Sportsman told the paper that the letter is valuable evidence, but that it was written in an attempt to mislead investigators.

"There is absolutely no reason to believe there have been any other killings or that any other killings have occurred related to this so-called confession," Sportsman told The Observer.

On Friday, Touma's body was sent to the Armed Forces Institute of Pathology in Washington, D.C., for a second autopsy, a police statement said.

In five years with the Army, Touma had served with the Army Dental Activity Clinic in Bamberg, Germany, and in Fort Drum, New York, before her assignment to Fort Bragg.